In the pulp and paper art it is highly desirable to improve the steam economy of the flash tanks utilized (which flash tanks are, for example, shown per se in U.S. Pat. Nos. 5,172,867, 4,551,198, and 5,700,355, the disclosures of which are hereby incorporated by reference herein). It may be possible to get such improved economy by decreasing chip bin or flash tank pressure to be able to get more flash steam from the flash tank. [Though the term xe2x80x9cflash tankxe2x80x9d is used throughout this discussion and is a term of the art, it is to be understood by those familiar with the art that this term includes any apparatus in which a hot pressurized liquid is exposed to a lower pressure and allowed to evaporate, typically violently, in an enclosed container to produce a source of steam and liquid at a lower temperature and pressure.] Black liquor could be directly flashed for example to a temperature 90xc2x0 C. instead of 107xc2x0 C. Then there would not be need for additional cooling of black liquor and also evaporation loading would slightly decrease. For example, the sub-atmospheric pressure in a flash tank could be maintained by a vacuum pump, such as shown in patent publication WO 97/29236.
However, there is a better solution. One can get more flashed steam from a flash tank by using a steam jet ejector. For example in some continuous digesters it is normal to use much low pressure (LP) steam for steaming even in the summer time. By using some LP steam in a steam jet ejector one can produce more flashed steam and the total LP steam consumption would decrease. A steam jet ejector is a very simple device without any moving parts. The lower LP steam consumption utilizing a steam ejector may give hundreds of thousands of dollars of savings per year for continuous digesters. The investment cost of the ejector should be less than 20% of that and there are essentially no additional operating costs.
There are also some other ways to use a steam jet ejector to improve energy efficiency in the digester area. For example LP steam pressure could be increased by medium pressure (MP) steam to be able to use the LP steam in digester heaters or a digester steam phase.
A steam jet ejector (as seen in FIG. 1) is a venturi jet device that uses the energy available in steam to either a) create a vacuum, b) boost the pressure of a gas, or c) a combination of a) and b). Single stage ejectors can be used to create vacuum levels of about 75 torr (1.5 psia), when discharging to atmosphere. Steam ejectors per se in the pulp and paper art are disclosed in U.S. Pat. Nos. 5,139,620 and 4,692,214. In co-pending application 09/195,444 filed on Nov. 18, 1998 [attorney. ref. 10-1268] a jet ejector is used to increase the efficiency of a spent cooking chemical heat recovery system having reboilers.
Black liquor is normally flashed in one or several stages against atmospheric pressure. Flashed steam is typically used to heat and expel air from the chips arriving to the process. This can be done for example in an atmospheric steaming vessel such as a DIAMONDBACK(copyright) chip bin (available from Ahistrom Machinery Inc. of Glens Falls, N.Y.), or in a conventional pressurized steaming vessel. Typically flashed steam is not enough to completely steam the chips and fresh low pressure steam is needed to complete steaming. Due to some friction losses and increased boiling point, black liquor temperature is typically about 107xc2x0 C. after flashing. Before sending the flashed black liquor to an evaporation plant, the black liquor is typically cooled in a heat exchanger by water to temperature of about 90xc2x0 C.
By using a conventional steam jet ejector (see FIG. 1 herein) to enhance the use of flash steam in an atmospheric steaming vessel, the flash tank pressure could be decreased to a pressure of about 0.5-1.1 bar absolute (abs.) pressure, preferably about 0.7-1.0 bar abs., and the black liquor would then be flashed to a temperature of about 80-102xc2x0 C., preferably about 90-100xc2x0 C. This way the total amount of usable flashed steam would increase and the use of valuable fresh steam could be decreased. The xe2x80x9chigh pressurexe2x80x9d steam utilized as a motive fluid of the steam jet ejector could be low pressure fresh steam, or flashed steam at a higher pressure from one or more previous flashing stages, or high pressure fresh steam. Additional benefits are that there is no longer a need to cool the black liquor going to the evaporation plant and the amount of black liquor to be evaporated would be slightly lower.
A steam jet ejector could also be used in other environments in a Kraft (or other chemical pulping) cooking plant:
Flashed steam pressure could be increased to be able to use it in a pressurized steaming vessel.
Flash tank pressure could be decreased to get more steam to a flashed steam condenser to produce more hot water.
Flashed or fresh low pressure steam pressure could be increased by higher pressure steam to be able to use it in the liquor heaters of a continuous digester.
Flashed or fresh low pressure steam pressure could be increased by higher pressure steam to be able to use it in the steam phase of a continuous digester.
Low pressure steam pressure could be increased by higher pressure steam to be able to use it in the direct or indirect liquor heaters of a batch digester.
The broadest embodiment of the invention comprises a method of treating hot spent cooking liquor, having a first pressure and a first temperature, using a flash tank, having a high-pressure liquid inlet, a low-pressure liquid outlet, and a steam outlet; and an ejector, having a high-pressure gas inlet, a low-pressure gas inlet, and a gas discharge, to recover energy from the liquor, consisting of or comprising: (a) introducing the hot spent cooking liquor at the first pressure into the high-pressure liquid inlet of the flash tank; (b) exposing the liquor in the flash tank to a second pressure, lower than the first pressure, so that at least some of the liquor evaporates to form steam and a cooler liquid at about the second pressure and at about a second temperature, lower than the first temperature; (c) removing at least some of the steam from the flash tank in a first gaseous stream; (d) introducing the first gaseous stream to the low-pressure inlet of the ejector; (e) introducing a second gaseous stream having a third pressure, greater than the second pressure, to the high-pressure inlet of the ejector; and (f) discharging a third gaseous stream at a fourth pressure, higher than the second pressure, from the discharge outlet of the ejector; and wherein (a)-(f) are practiced so that the second pressure in the flash tank is lower (e.g. by at least about 0.1 bar abs) than the pressure that would be present in the flash tank without the presence of the ejector under otherwise substantially identical conditions. The present invention also includes practicing (a)-(f) so that the second pressure and the second temperature are lower than the pressure and temperature would be in the prior art without the presence of the ejector under otherwise substantially identical conditions.
The method as recited above may further comprise (h) discharging concentrated hot spent cooking liquor from the flash tank at a temperature at least 2xc2x0 C. lower than would be present without the utilization of the ejector under otherwise substantially identical conditions.
The hot liquor is preferably hot spent extraction liquor removed from a kraft pulping process, for example, a continuous or batch pulping process. The liquor typically has a temperature of between about 100xc2x0 C. to 180xc2x0 C., preferably between about 110xc2x0 and 160xc2x0 C., that is, about the temperature of the pulping process, and a pressure ranging from about 5 to 15 bar gage (that is, 6 to 16 bar abs.)
The present invention may also further include, prior to (a), (g) cooling the liquor from the first temperature to a third temperature, lower than the first temperature (e.g. by at least about 5xc2x0 C.). The cooling process (g) is preferably practiced by passing the hot liquor in heat exchange relationship with a process liquid associated with the cooking process, for example, cooking chemical, such as kraft white, green, or black liquor, or dilution liquor or filtrate (for example, cold blow filtrate, washer filtrate or bleach plant filtrate). This cooler liquid typically has a temperature of less than 130xc2x0 C. and is typically at between about 60 and 120xc2x0 C., preferably between about 80 and 100xc2x0 C.
The second pressure in the flash tank is typically less than 4 bar absolute, for example, between about 0.1 to 2 bar abs., preferably between about 0.5 to 1.5 bar abs. The second temperature in the flash tank typically corresponds to the saturation temperature of the gas (typically xe2x80x9cdirtyxe2x80x9d steam) generated by the exposure of the hot liquor to the second pressure. For example, when the second pressure is between about 0.5 to 1.5 bar abs., the second temperature is between about 80 and 110xc2x0 C. The second gaseous stream, having a third pressure higher than the second pressure, introduced to the high-pressure inlet of the ejector is typically any available gas stream that will induce a lower pressure in the low-pressure inlet of the ejector. This second gaseous stream is preferably any flow of steam that is readily available in the pulp mill having any available third pressure. For example, the second gaseous stream may be xe2x80x9clow-pressure steamxe2x80x9d in which the third pressure may range from about 3.5 to 4.5 bar gage (that is, about 4.5 to 5.5 bar abs. or about 50 to 65 psig) or xe2x80x9cmedium-pressure steamxe2x80x9d in which the third pressure may range from about 10 to 12 bar page (that is, 11 to 13 bar abs. or about 145 to 175 psig) or even xe2x80x9chigh pressure steamxe2x80x9d in which the third pressure may range from 13 to 100 bar gage (that is, 14 to 101 bar abs. or about 200 to 1500 psig). The temperature of the second gaseous stream may be between about 100xc2x0 to 250xc2x0 C., but is typically, between about 140xc2x0 to 160xc2x0 C. Though the second gaseous stream is preferably xe2x80x9ccleanxe2x80x9d steam, for example, steam having little or no sulfur compounds, according to the invention, the second gaseous stream may also be xe2x80x9cdirtyxe2x80x9d steam. This steam may also be xe2x80x9cfreshxe2x80x9d steam.
The fourth pressure of the third gaseous stream discharged from the ejector during (f), which according to the present invention is greater than the second pressure and lower than the third pressure, is typically a function of the magnitude of the second pressure and the third pressure. The fourth pressure may typically vary from about 0.5 to 5 bar abs., but is preferably between about 0.8 to 3 bar abs. This fourth pressure, according to the present invention, is typically about the same as or less than the pressure of the gas stream released from a prior art flash tank without the use of an ejector, although in some circumstances the fourth pressure may also be greater than the pressure of the gas stream released from a prior art flash tank under otherwise substantially identical conditions.
According to the present invention, the use of the ejector to reduce the pressure inside a flash tank and then raise the pressure supplied to other equipment or processes, typically allows the flash evaporation of hot liquors to produce steam having lower temperatures and greater volumes than the prior art. The lower pressure also permits the lowering of the temperature of the cooled liquor sent, for example, to evaporation. For example, the temperature of the steam and liquid in the flash tank according to the present invention may be lowered at least about 2xc2x0 C., typically at least about 5xc2x0 C., preferably at least 10xc2x0 C. compared to the prior art. At the same time, the volume of gas (steam) produced may typically be increased by at least about 10%, preferably at least 20%, sometimes even more than 40% compared to the prior art.
The present invention may further include (h) discharging the cooler liquid formed at (b) from the low-pressure liquid outlet of the flash tank. The liquid discharged from the low-pressure outlet of the flash tank will typically have a temperature about equal to the second temperature and a pressure about equal to the second pressure. The temperature of this cooler liquid may be below 135xc2x0 C., typically, below 110xc2x0 C., preferably below 100xc2x0 C. This liquid is may pass through two or more additional flash tanks, with or without ejectors according to the invention, and is typically forwarded to the recovery process. This cooler liquid may also be returned to the pulping process for treating the cellulose material prior to or during the pulping process.
The present invention also includes a method of treating hot spent cooking liquor having a plurality of flash tanks having one or more ejectors wherein (a) through (f) are practiced in association with one or more of the flash tanks, preferably, with at least the last or final flash tank. The present invention may also include a plurality of flash tanks, one or more jet ejectors, and one or more heat exchangers wherein (a) through (f) are practiced in one or more flash tanks and (g) is practiced at least before the first flash tank.
Another embodiment of this invention comprises a method of treating a first gaseous stream having a first pressure in a pulp mill to produce a second gaseous stream at a second pressure, higher than the first pressure, using a jet ejector having a high-pressure inlet, a low-pressure inlet, and a discharge outlet, consisting or comprising: (a) introducing the first gaseous stream having the first pressure to the high-pressure inlet of the jet ejector; (b) introducing the second gaseous stream to the low-pressure inlet of the jet ejector; and (c) discharging a mixture of the two gaseous streams to form a third gaseous stream which is discharged from the discharge outlet at a third pressure, greater than the second pressure. The first gaseous stream is preferably a medium pressure (MP) or a high-pressure (HP) steam having a pressure greater than 5.0 bar gage, typically greater than 10 bar gage. The second gaseous stream is preferably low-pressure (LP) steam at a pressure of about 2.5 to 5.0 bar gage (that is, about 3.5 to 6 bar abs. or about 35 to 75 psig). The third pressure is a function of the first and second pressures and is typically between about 3 and 10 bar gage, preferably between about 4 and 9 bar gage, most preferably between 5 and 8 bar gage.
A preferred embodiment further includes (d) monitoring the third pressure and controlling the first pressure in response to the monitoring of the third pressure. This is typically practiced using a conventional automated control loop.
The third gaseous stream at the third pressure may be forwarded to other processes in the pulp mill as needed, including, to heat exchangers for heating other fluids, to a steam-phase digester as the source of steam, to a condenser to improve the efficiency of the evaporators, to one or more batch digesters for heating,
Another embodiment of the present invention comprises a system for treating hot spent cooking chemical to recover energy comprising: A source of hot spent cooking liquor. A flash tank having a hot liquid inlet operatively connected to the source of hot spent cooking liquor, a cooled liquid outlet, and a steam outlet. A jet ejector having a high-pressure inlet, a low-pressure inlet operatively connected to the steam outlet of the flash tank, and a discharge for mixed steam. A source of pressurized fluid operatively connected to the high-pressure inlet of the jet ejector. And, means for using the mixed steam discharged from the jet ejector operatively connected to the discharge of the jet ejector.
The source of spent cooking chemical is preferably a chemical digestion process as described above. The flash tank is preferably a conventional flash tank as described in the above reference patents and provided by Ahistrom Machinery Inc. of Glens Falls, N.Y. The jet ejector is preferably a conventional ejector or xe2x80x9cthermocompressorxe2x80x9d which can handle the temperatures, pressures described above. One preferred thermocompressor is a Graham Thermocompressor manufactured by Graham Manufacturing of Batavia, N.Y., though comparable thermocompressors, eductors, or jet ejectors may be used. The thermocompressor is preferably made from steel, preferably stainless steel, for example, 300-series stainless steel or its equivalent.
The source of pressurized fluid is any source typically available in a pulp mill, for example, low-pressure steam, medium-pressure steam, or high-pressure steam as described above. This steam may be xe2x80x9cfreshxe2x80x9d steam and/or xe2x80x9ccleanxe2x80x9d steam.
The means for using the mixed steam discharged from the jet ejector may be any steam utilization device or process available in the pulp or paper mill. Some preferred uses include for steaming of wood chips and the like in chip bins or steaming vessels; heating in direct or indirect heat exchangers, for example, in cooking circulation associated with a digester; in steam-phase digesters as the source of steam; or the like.
The system may also include means for cooling the hot liquor positioned between the source of hot liquor and the flash tank for cooling the hot liquor prior to introducing it into the flash tank. The cooling means may be one or more direct or indirect heat exchangers, or any other conventional equipment capable of performing the cooling function. One preferred heat exchanger is an Extraction Liquor Cooler provided by Ahistrom Machinery Inc. (though any conventional heat exchanger can be used), which heat exchanger is typically provided with a source of cooling medium, for example, liquors related to the cooking process as described above.
The present invention may also include two or more flash tanks having high-pressure liquid inlets, low-pressure liquid outlets, and steam outlets. A jet ejector may be positioned in one or more of the steam outlets of the flash tanks. In a preferred embodiment, the invention includes a plurality of flash tanks and the jet ejector is positioned in the steam outlet of the last or final flash tank. When a plurality of jet ejectors are used with a plurality of flash tanks one or more sources of steam may be used as the motive fluid in the one or more jet ejectors. For example, medium-pressure steam (e.g. about 12 bar abs.) may be used as the motive fluid in a first ejector and low-pressure steam (e.g. about 4.5 bar abs.) may be used for another second ejector. The mixed higher-pressure steam discharged from the first ejector may be forwarded to means for using the steam (e.g., a digester feed system steaming vessel), and the mixed lower-pressure steam from the second ejector may be forwarded to another means for using the steam (e.g., the chip bin of a digester feed system). The two or more ejectors may be provided with the same source of motive fluid (e.g., medium-pressure steam at 12 bar abs. or low pressure steam at about 5 bar abs.) The steam provided may be xe2x80x9cfreshxe2x80x9d steam and/or xe2x80x9ccleanxe2x80x9d steam. Also, the steam discharged from the steam outlet of one flash tank may also provide the motive fluid introduced to the high-pressure inlet of one of the one or more jet ejectors. The present invention may also include one or more heat exchangers located upstream of each of the plurality of flash tanks.
It is the primary object of the present invention to enhance the amount of steam produced from flash tanks in a conventional chemical pulping process and/or decrease the amount and temperature of spent cooking liquor discharged from a flash tank, and/or increase the concentration of the black liquor discharged from a flash tank in a chemical pulping system, and/or to increase the pressure of a low pressure steam flow in a pulp mill, all in a simple yet effective manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.